Rhodospirillum rubrum grows anaerobically in light, or fermentatively in darkness with pyruvate. Fermenting cells resemble light-grown R. rubrum by containing "chromatophores". Nevertheless, these chromatophores probably do not photophosphorylate, since light-induced oxidation of cytochrome c2 could not be demonstrated. The inability to photophosphorylate could help explain the diauxic growth response observed when anaerobic, dark-grown R. rubrum was placed into pyruvate medium under low-light (860 lux) conditions. In experiments, once pyruvate was fermented, the cells entered a lag period, followed by photosynthetic growth. The duration of the lag period was inversely related to light intensity and could be abolished by exposing the cells to saturating light. Consequently, it appeared that during fermentative growth, high light might render the "dark-formed" membranes photosynthetically competent. To test this, the effect of radiant energy on the appearance of cytochrome c2 oxidation and anaerobic photophosphorylation activities is being examined. In separate studies, it was observed that after R. rubrum had developed photosynthetic competence, each time the cells were transferred into pyruvate medium in the light, they exhibited diauxie. The observation that cyclic-3',5'-adenosine monophosphate alters the diauxie in a predictable way has suggested that catabolite repression, or a closely related process, is involved.